The traditional Marx generator, named for its inventor, Professor Erwin Marx, produces a single, high-voltage pulse by switching precharged capacitors into a series-connected string using gas-insulated spark gaps. The Marx generator is a rugged, low-impedance source of electrical energy that has served well in a wide variety of high-peak-power applications for the past 75 years.
Marx generators are now undergoing a renaissance of new applications with the use of modern solid-state switches. The use of solid state switches with current interruption capability in place of spark gaps, for example, gives simple Marx generators the ability to produce square-shaped output pulses at very high rates. Some examples of solid-state switches with current interruption capability include the Bipolar Junction Transistor (BJT), Field Effect Transistor (FET), and Insulated Gate Bipolar Transistor (IGBT), and Gate Turn Off (GTO). The current interruption capability of these switches also allows the output pulse to change width from one pulse to the next, a capability that gives the generator the ability to adapt rapidly to changing load requirements.
Currently, Marx generators using solid-state switches are unable to equal the high peak voltage and peak power capacity of generators using spark gaps, but the operational advantages gained in pulse control and high average power have transformed the single-shot Marx generator into a versatile modulator. The present invention embodies the next step in development of the solid-state Marx modulator, by replacing the usual inductive or resistive charging elements with fast-recovery diodes, as shown in FIGS. 1 and 2.
Krasnykh was the first to report using IGBT switches in a Marx-style modulator with resistive charging elements [A. Krasnykh, R. Akre, S. Gold, and R. Koontz, “A Solid-State Marx Type Modulator for Driving a TWT,” Conference Record of the 24th International Power Modulator Symposium, 2000, p. 209.] Krasnykh also mentions the use of diodes to replace one of the two resistor charging strings. Okamura was the first to report using all diode-directed charging in a solid-state Marx switched by thyristors [K. Okamura, S. Kuroda, and M. Maeyama, “Development of the High Repetitive Impulse Voltage Generator Using Semiconductor Switches,” Proceedings of the 12th IEEE International Pulsed Power Conference, 1999, p. 807.] The diodes route pulse-charging current from a command-resonant charging system to the energy-storage capacitors at rates up to 2 kHz. In this case, the thyristors have no current interruption capability and therefore do not offer pulse width selection.
Gaudreau and Casey describe an IGBT-switched Marx modulator design (for which they presented no test data) that uses diodes in place of charging elements [Marcel M. P. Gaudreau, et al., “Solid-State Pulsed Power Systems for the Next Linear Collider,” Digest of Technical Papers for the Pulsed Power and Plasma Science Conference, PPPS-2001, 2001, p. 289.] The Gaudreau and Casey circuit design showed an internally distributed charging inductor in parallel with one of the diode strings. This arrangement does not recycle energy, and no mention was made of pulse shape control.
In a later publication, Casey described tests of an IGBT-switched Marx modulator that used common-mode isolation inductors as charging elements and a single string of freewheeling diodes. The isolation inductors provided a low-impedance path for difference currents, such as charging or filament currents, and a high impedance path to common-mode currents during the Marx output pulse.
In a recent publication, Jeffery A. Casey, et al., (Abstract) “Solid-state Marx Bank Modulator for the Next Generation Linear Collider,” Conference Record of the 26th International Power Modulator Symposium and 2004 High Voltage Workshop (PMC), San Francisco, Calif., May 23-26, 2004, Casey describes the ability of the freewheeling diodes to pass the load current around an inactive Marx stage. By doing so, additional stages may be added to the Marx assembly and fired at staggered intervals to compensate for the natural pulse droop. Modeling data indicates the efficacy of staggered stage switching, but no demonstration of staggered switching was provided. Richter-Sand also reported development of an IGBT-switched Marx modulator that used common-mode isolation inductors for charging and develops each stage voltage across a diode [R. J. Richter-Sand, et al., “Marx-Stacked IGBT Modulators for High Voltage, High Power Applications,” Conference Record of the 25th International Power Modulator Symposium and 2002 High Voltage Workshop (PMC), 2002, p. 390]. However, as in the prior cases, no mention was made of pulse shape control or energy recovery.
Other researchers have elected to use pulse-forming networks (PFN) in place of simple capacitors to shape the Marx output pulse and thereby eliminate the need for opening switches or additional energy storage. Since no more energy is stored than is needed by a single pulse, the PFN assembly is very compact, but at the sacrifice of pulse flexibility.
Other researchers have developed non-Marx-type solid-state modulators with pulse-width and pulse-shape agility. These modulators, induction voltage adders (IVA), use transformers to sum potentials contributed by independent sources. The primary winding on each independent transformer is powered by a solid-state source. A secondary winding, common to all the transformers, collects the power from each independent source and algebraically sums their potentials. A major advantage of IVAs is that all the sources are controlled from the ground potential, with the transformer providing the high-voltage isolation.
Kirbie developed the first solid-state IVA to power induction accelerators. Cook and Watson developed a much faster IVA to power an electron-beam kicker, along with control methods for pulse-shape agility, [H. Kirbie, et al., (Invited) “An All Solid-State Pulse Power Source for High PRF Induction Accelerators,” Conference Record of the 23th International Power Modulator Symposium, 1998, p. 6]. Commercial IVAs are also available from First Point Scientific. All the IVAs require a transformer for each source, and the transformer includes magnetic core material that significantly increases assembly weight and, because of magnetic saturation, limits the maximum pulse width.
Most of the advantages of our Marx modulator come from our use of diodes as charging elements. While other researchers have used diodes as well as IGBTs in a variety of solid-state modulators, including Marx-style modulators, we believe no one has taken our approach or demonstrated similar benefits
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.